We should be clear that this would not require development of a ‘new’ nuclear weapon because it would be based on an existing weapon and a long-standing military requirement. Such an approach would be consistent with the policy of the Obama administration that stockpile modernization will ‘use only nuclear components based on previously tested designs, and will not support new military missions or provide for new military capabilities.’

Even so, our recommended effort might still evoke comparisons with the Bush administration’s controversial ‘Robust Nuclear Earth Penetrator’ study. To allay these concerns, however, the United States should make clear that, should an underground nuclear explosive test be necessary to confirm the reliability of a modified B83, it would look for other options to meet this military requirement.

Our proposal wouldn’t threaten strategic stability with Moscow or Beijing because of both the number and the nature of the weapons. First, the number of converted penetrators would be kept small, consistent with a niche capability against North Korea, which means they couldn’t threaten the redundant and variegated forces of Russia or China. In any event, we doubt Russia or China would be genuinely threatened by such an effort. Russian officials didn’t object to the development of the B61-11, even though that penetrator was clearly intended to defeat Russian targets in frozen tundra. Indeed, it’s also likely that many important Russian facilities are much too hard for even nuclear weapons.

Nor would our proposal make the B83 ‘more usable’ or lower the threshold for nuclear use. Such a penetrator could only be used, as the 2010 Nuclear Posture Review states, in extreme circumstances. Consistent with that reality, the United States should make clear that it would also invest heavily in conventional penetrators and functional defeat approaches that would provide the president with credible, indeed more desirable, non-nuclear options for holding at risk targets in hard rock. Kim will only be more cautious if he knows that the president has a full suite of options, including non-nuclear ones, to get at him wherever he may hide. Until then, however, his realization that he isn’t immune may help him reconsider some of his more reckless behaviour.

Our first priority at this moment is to ensure the security of allies, particularly Japan and South Korea. It is our security umbrella that allows each state to remain non-nuclear even in the face of North Korea’s provocative behaviour, including its testing and development of nuclear weapons. Since the end of the Cold War, however, the United States has deemphasized its tactical nuclear weapons, affirming that ‘it has no nuclear weapons on the Korean Peninsula’ in the 2005 Six Party Agreement and announcing plans to retire the nuclear-armed Tomahawk missile by 2013. In response to North Korea's provocative behaviour, calls have arisen in Japan for an independent nuclear capability and in South Korea for the emplacement of US tactical nuclear weapons. Neither of these would be positive developments, but they reflect genuine and reasonable security concerns. Compared to these outcomes, a new casing for the B83 seems a reasonable response to North Korea’s dangerous behaviour.

Kim Jong-il’s aggressive behaviour demonstrates the continuing importance of deterrence. The best US response is to maintain capabilities that clearly demonstrate to him that the United States has a credible and effective option to hold at risk what he values most, while preserving stability and respecting our international non-proliferation commitments. Such a capability would help encourage Kim to err on the side of caution when contemplating provocations that might escalate, such as the attacks on the Cheonan and Yeonpyeong. A modified B83 is just such a capability.

Jeffrey Lewis is Director of the East Asia Nonproliferation Programme at the James Martin Center for Nonproliferation Studies at the Monterey Institute of International Studies. He also founded and maintains the leading blog on nuclear arms control and nonproliferation, ArmsControlWonk.com. Before joining CNS, he was Executive Director of the Nuclear Strategy Initiative at the New American Foundation. Elbridge Colby is a research analyst at the Center for Naval Analyses, where he focuses on strategic, deterrence, proliferation, and related issues. From 2009 to 2010, he served as policy advisor to the Secretary of Defence’s Representative for the follow-on to the Strategic Arms Reduction Treaty.